Copolyester polymers have been extensively used in elastomeric springs or pads installed between a pair of metal plates for absorbing energy. One such polymer is manufactured under trademark HYTREL by E.I. DuPont de Nemours & Co. of Wilmington, Del. As is well known, the general method of making the spring first begins with the step of processing such polymer into what is commonly referred to as a block or a preform of a substantially solid body and then with the step of precompressing the block or preform into a generally disk-shaped spring or pad under axial forces applied thereto an extent greater than thirty percent of its original axial height. After the axial precompression force is removed, the spring recovers to its normal operating height (thickness) which is smaller than the original height of the preform but is greater than the precompressed height. The precompressed height is also commonly referred to as a “solid height”. U.S. Pat. No. 4,198,037 issued to Anderson illustrates one type of such method wherein the preform is casted as a cylindrical block having a central projection formed on one axial end and a central socket formed on an opposed axial end. Anderson is particularly concerned about operability of such copolyester material in a compression mode and finds that the optimum precompression rate of the original block or preform is about fifty percent. The spring is then positioned between a pair of metal plates and precompressed again to interlock the socket and projection with complimentary elements provided within the metal plates in order to form the final spring operable for absorbing the energy. In U.S. Pat. No. 5,351,844 Carlstedt provides a preform which is molded from an elastomer with a raised projection at each axial end.
However, as there has been a continuing desire for increasing the energy that can be absorbed by the spring, particularly when the spring is used on railway vehicles, there is a need for additional improvements in a method of making compressible elastomeric springs.